Inkjet printing is typically done by either drop-on-demand or continuous inkjet printing. In drop-on-demand inkjet printing ink drops are ejected onto a recording medium using a drop ejector including a pressurization actuator (thermal or piezoelectric, for example). Selective activation of the actuator causes the formation and ejection of a flying ink drop that crosses the space between the printhead and the recording medium and strikes the recording medium. The formation of printed images is achieved by controlling the individual formation of ink drops, as is required to create the desired image.
Motion of the recording medium relative to the printhead during drop ejection can consist of keeping the printhead stationary and advancing the recording medium past the printhead while the drops are ejected, or alternatively keeping the recording medium stationary and moving the printhead. This former architecture is appropriate if the drop ejector array on the printhead can address the entire region of interest across the width of the recording medium. Such printheads are sometimes called pagewidth printheads. A second type of printer architecture is the carriage printer, where the printhead drop ejector array is somewhat smaller than the extent of the region of interest for printing on the recording medium and the printhead is mounted on a carriage. In a carriage printer, the recording medium is advanced a given distance along a medium advance direction and then stopped. While the recording medium is stopped, the printhead carriage is moved in a carriage scan direction that is substantially perpendicular to the medium advance direction as the drops are ejected from the nozzles. After the carriage has printed a swath of the image while traversing the print medium, the recording medium is advanced; the carriage direction of motion is reversed; and the image is formed swath by swath.
A drop ejector in a drop-on-demand inkjet printhead includes a pressure chamber having an ink inlet for providing ink to the pressure chamber, and a nozzle for jetting drops out of the chamber. Two side-by-side drop ejectors are shown in prior art FIG. 1 (adapted from U.S. Pat. No. 7,163,278) as an example of a conventional thermal inkjet drop on demand drop ejector configuration. Partition walls 20 are formed on a base plate 10 and define pressure chambers 22. A nozzle plate 30 is formed on the partition walls 20 and includes nozzles 32, each nozzle 32 being disposed over a corresponding pressure chamber 22. Ink enters pressure chambers 22 by first going through an opening in base plate 10, or around an edge of base plate 10, and then through ink inlets 24, as indicated by the arrows in FIG. 1. A heater 35, which functions as the actuator, is formed on the surface of the base plate 10 within each pressure chamber 22 and is configured to selectively pressurize the pressure chamber 22 by rapid boiling of a portion of the ink in order to eject drops of ink through the nozzle 32.
FIG. 2 shows a prior art configuration of drop ejectors 60 disposed as a linear array 52 along an array direction 54 on a printhead 50. For simplicity, only the pressure chamber 22 and the nozzle 32 are shown for each drop ejector 60. The spacing between drop ejectors 60 in linear array 52 along array direction 54 is Dy. Recording medium 62 and printhead 50 are moved relative to each other along scan direction 56, and drop ejectors 60 are controllably fired to eject drops of ink toward recording medium 62. Dots are formed on recording medium 62 where ink drops land. Allowable image dot locations 66 are defined by a pixel grid 64 including pixel rows 68 and pixel columns 70. The spacing of pixel columns 70 from each other along the array direction is Dy, which is the same as the spacing between drop ejectors 60 in linear array 52. The spacing Dx of pixel rows 68 from each other along the scan direction 56 is related to the timing of firing of drop ejectors 60. For recording medium 62 and printhead 50 moving at constant velocity V relative to each other along scan direction 56, Dx=Vt=V/f, where t is the time interval between consecutive firings of drop ejectors 60 and f is the drop ejection frequency. For many types of printheads 50, drop ejectors 60 cannot be all fired simultaneously due to excessive electrical current requirements. In such cases, the linear array 52 is typically not actually a straight line. Rather the drop ejectors 60 are offset as needed in order to compensate for firing at different times so that the ink drops land along substantially straight pixel rows 68 on recording medium 62.
Image resolution Rx along the scan direction 56 is equal to 1/Dx=f/V. In other words, the print speed V=f/Rx. For a desired image resolution along the scan direction, Rx is proportional to the drop ejector frequency f and inversely proportional to print speed. There are physical limitations to the drop ejection frequency f. For example, the pressure chamber 22 needs to refill with ink before a subsequent drop can be fired.
Image resolution Ry along the array direction 54 is equal to 1/Dy. For a linear array 52, in order to have a high resolution Ry, the drop ejector spacing Dy needs to be small. Drop ejectors 60 of various types need to have a certain size to eject sufficiently large drops in order to provide good ink coverage on the recording medium 62. A typical achievable drop ejector spacing Dy for a thermal inkjet drop ejector is 42.3 microns, equivalent to 600 nozzles per inch. By contrast, a typical achievable drop ejector spacing for a piezo inkjet printhead is approximately 254 microns, equivalent to 100 nozzles per inch. Conventional thermal inkjet printheads can provide 1200 spot per inch resolution Ry by providing two staggered linear arrays 52 of drop ejectors 60.
In order to enable high resolution printing for larger drop ejectors, such as piezo drop ejectors, multiple offset rows of drop ejectors can be provided on a printhead, as seen in prior art FIG. 3 adapted from U.S. Pat. No. 7,300,127. Rows of drop ejectors extend horizontally along array direction 54 in FIG. 3. Each drop ejector in the figure includes a pressure chamber 102 and a nozzle 100-kl, where l indicates the row number with the first row (l=1) being at the bottom, and k indicates the position within each row and increases toward the right. A first row of drop ejectors includes nozzles 100-11, 100-21, 100-31. A second row of drop ejectors includes nozzles 100-12, 100-22 (not labeled) and 100-32 (not labeled). The second row is offset along the array direction 54 from the first row by a distance P. There are a total of six rows, so the spacing in the array direction 54 between nozzle 100-11 and 100-21 is 6P. By appropriately timing the firing of drop ejectors as the recording medium is moved relative to the printhead, the drops can be made to land on the recording medium to form dots in a horizontal line along the array direction 54 as shown. The leftmost dot in FIG. 3 was ejected by nozzle 100-11. The adjacent dot to the right (shown as being located a distance P to the right of the leftmost dot) was ejected by nozzle 100-12. Using such a two-dimensional “staggered lattice” of drop ejectors, high resolution printing can be provided even though individual drop ejectors are large compared to the dot spacing P. As the recording medium is moved relative to the staggered lattice of drop ejectors in the scan direction 56, additional horizontal lines of dots can be printed.
Even for compact types of drop ejectors such as thermal inkjet drop ejectors, it can be beneficial to arrange the drop ejectors in multiple offset rows in order to provide room for ink feeds and electrical circuitry, as shown in prior art FIG. 4 adapted from U.S. Pat. No. 8,118,405. Printhead module 210 (shown in a top view in FIG. 4) is one of a plurality of printhead modules 210 that are assembled together end to end at butting edges 214 in order to extend the printhead length. Arrays 211 of drop ejectors 212 are inclined relative to the non-butting edges 209 of printhead module 210. Ink can be fed from the back side of printhead module 210 through segmented ink feeds 220 including slots 221 that extend from the back side to the top side. Ink then flows from slots 221 to ink inlets 24 (FIG. 1) to enter pressure chambers 22 (FIG. 1) of the drop ejectors 212. The segmented ink feeds 220 are disposed adjacent to arrays 211 of drop ejectors 212. Also disposed between arrays 211 and near butting edges 214 is electrical circuitry 230 that can include driver transistors to provide electrical pulses for firing drop ejectors 212, as well as logic electronics to control the driver transistors so that the correct drop ejectors 212 are fired at the proper time. Electrical contacts 240 extend along one or both non-butting edges 209 for providing electrical signals to the electrical circuitry 230. Recording medium (not shown) is advanced relative to printhead module 210 along scan direction 56.
A plurality of printheads having corresponding nozzles that are aligned to each other can be used to form dots having multiple ink drops per dot, as shown in FIGS. 5A and 5B adapted from Japanese Patent Application Publication No. 10-151735 (JP '735). Printheads 2 and 4 are mounted on a common carriage (not shown) that is moved along scan direction 56. Corresponding nozzles 18 in printheads 2 and 4 are aligned along the scan direction 56. The drop ejectors are sized such that ejected drops have half the drop volume required to form a dot of the desired size on the recording medium. FIG. 5A shows half-sized dots 40 that are printed by only the nozzles 18 in printhead 2. FIG. 5B shows overlapping dots formed by nozzles 18 on both printheads 2 and 4. A more generalized example disclosed in Japanese Patent Application Publication No. 10-151735 is the use of three or more printheads having aligned nozzles 18, where the drop ejectors are sized to provide drop volumes that are inversely proportional to the number of printheads. An advantage stated is that the printing speed can be increased.
A plurality of printheads having corresponding nozzles that are aligned to each other is also disclosed in Japanese Patent Application Publication No. 10-157135 (JP '135). In JP '135 two printheads each having a single row of drop ejectors are arranged in similar fashion to FIG. 5A adapted from JP '735. In JP '135 aligned drop ejectors of the two printheads are controllably fired to form dots on a scan line from each printhead in order to compensate for drop volume nonuniformity of drop ejectors on the two printheads.
Drop ejectors can fail during the life of a printer. For example there can be electrical failure of the actuator, such as a failed resistive heater in a thermal inkjet drop ejector. Alternatively a drop ejector nozzle can become plugged. For inkjet printheads (such as those in FIGS. 2 through 4) that print in a single pass and that have a single drop ejector responsible for printing all pixels on a line along the scan direction 56, a non-recoverable failure of a single drop ejector results in an objectionable white streak in the image along the scan direction 56. Carriage printers can disguise the effects of failed drop ejectors through multi-pass printing where each printed line of dots along the carriage scan direction is printed by multiple drop ejectors during the multiple print passes where the recording medium is advanced along the scan direction between each pass. However, multi-pass printing reduces printing throughput dramatically.
Despite the previous advances in drop ejector configurations on inkjet printheads, what is still needed are printhead and printing system designs, as well as printing methods, that provide high resolution printing with high reliability and image uniformity, even if high speed single-pass printing is used and even if one or more drop ejectors fail